A typical magnetic disk drive digital storage system includes a read/write head system that controls the reading of information from and the writing of information to the magnetic disk. A servo circuit of the read/write head system controls the positioning of a read/write head relative to the disk when reading information from and writing information to the disk.
Prior to shipment of a disk drive digital storage system, a servo field is written on the disk in the factory. During normal read/write operations, the servo circuit analyzes the servo field and moves the read/write head so that it is positioned correctly in the center of the disk track to be read or written.
As a disk spins, it wobbles to some extent. This wobble is commonly referred to as track mis-registration. A predictable wobble is commonly referred to as repeatable runout. In order to correct for this repeatable runout, the extent of repeatable runout is measured in the factory and an embedded repeatable runout correction (ERRC) bit pattern is written to the disk as part of the servo field. During normal operations, the servo circuit analyzes the ERRC bit pattern read from the disk and moves the read/write head by an amount indicated by the ERRC bit pattern to correct for wobble and ensure that the read/write head is centered on the track it is reading or writing.
Although the addition of the ERRC bit pattern to the servo field provides an improvement to servo track following, a problem still exists in that an error in the ERRC bit pattern can cause the read/write head to be incorrectly positioned. In fact, an error in reading this pattern may introduce more position error than if no attempt was made to correct for the repeatable error. Current servo circuits perform an interrupt service request (ISR) algorithm that determines whether the ERRC value read is a reasonable value. However, even when the ERRC value read is determined to be reasonable, it may still be incorrect and result in the read/write head being moved an incorrect amount and/or in the wrong direction.
Accordingly, a need exists for a method and apparatus for detecting and correcting errors in an ERRC bit pattern of a servo field.